The government has failed to convince the public that coalitions are good for the country, and more than two-thirds of people now believe the government is weaker, less decisive and "confused" about what it stands for.

Only half of people believe the Liberal Democrats have had an influence in the coalition and most people think the Tories have got the best deal.

Nick Clegg should stop trying to be an overarching deputy prime minister and take on a major government department if he wants to improve his powerbase, the Institute for Government (IFG), which commissioned the poll, said.

"It is very difficult to exert serious political authority sitting in the Cabinet Office without a portfolio," Lord Adonis, the former Labour transport secretary and director of the IFG, said. The coalition had failed to convince the electorate of its efficacy, he said. "Their experience of coalition over the past year seems to have turned them decisively against it."

The IFG poll, part of a report One Year On – The First Year of Coalition Government, found that 63% of people think it is a bad thing that no party won a majority at the election – up from 55% a year ago. Some 68% think that coalition has created a weaker government, 73% believe government is less decisive and 80% say the government is "more confused" with a coalition.

Some 58% say the Liberal Democrats abandoned their principles by entering a coalition with the Tories and only 44% believe they have had a significant influence on government policy. However, a slight majority – 52% – said they believed the Liberal Democrats were right to enter the coalition. Even on constitutional reform – the Liberal Democrats' flagship policy which led to the alternative vote referendum – only 40% of people said they believed the party had had an influence.

Adonis said: "The polls show that the coalition is not viewed as a partnership of equals. A large majority believe the Liberal Democrats 'abandoned their principles' by forming a coalition with the Conservatives … As a political project, the coalition clearly needs to reinvent and renew itself, even more so following the AV [alternative vote] and local election votes.

"A significant pointer to the future will be whether the deputy prime minister revisits his decision not to take a more prominent role as secretary of state for one of the major departments or whether he sticks to constitutional reform as his major preoccupation."

The report also claims that "the Liberal Democrats have suffered as a result of failing adequately to preserve their distinct identity within the coalition".

Yesterday, Clegg promised to exert his party's independence more within the coalition. "I totally accept that, and this was one of the things that came through last week, that if you are a Conservative voter one of the things you are accustomed to is the Conservative party go into government and cut public spending," he said in a speech at the National Liberal Club.

"Some Conservative voters might even both demand it and like it. If you are a Liberal Democrat voter, you don't like it. I don't like it. I didn't come into politics to kind of announce great cuts and savings. You do it out of necessity."

David Cameron, meanwhile, last night issued a direct challenge to Clegg when he dismissed claims by the Liberal Democrats that they had used their influence to pause the legislation introducing the government's controversial NHS reforms, claiming it was a Tory decision.

The pollsters Populus interviewed 1,003 people by phone between April 15 and 17.